The U.S. National Park System, beloved by Americans and foreign visitors alike, is suffering the effects of popularity. Overcrowding, water and air pollution and vandalism have been rising throughout the 367-unit system. Conservationists say many parklands face such severe pressure that they need protection from additional recreational use and adjacent commercial development. Moreover, they say, park concessionaires and other private interests should pay higher fees to operate on public lands. But commercial interests seek greater access to federal mineral, timber and water resources as well as increased recreational use of the parks. In their view, natural resources can be used without debasing the parks' essential character. President Clinton, meanwhile, has proposed higher entry fees and new user charges to help the strained system.